Hollow glass articles are conventionally mass-produced by introducing a gob of molten glass during each operating cycle of a glass-blowing apparatus into a premold in which a parison is formed by blowing or pressing. At the end of the cycle, the parison is transferred to a final mold where it is blown into the ultimate shape of the desired article during an immediately following cycle in which a new parison is formed in the premold, the sequence being then repeated.
The length of an operating cycle is determined inter alia by the time required for the cooling of the parison to the proper temperature for the final blowing. If the parison is too hot during conversion into the finished article, the latter will tend to deform upon its extraction from the blow mold; if it is too cold, it may not attain its desired final shape in that mold. Thus, it is customary to transfer the parison from the premold to the final mold only after it has cooled down sufficiently and its temperature has been substantially equalized throughout its cross-section, such equalization being a relatively slow process because of the poor thermal conductivity of the glass.